1. Field of the Invention
The present invention relates to a method and apparatus for changing a Transmit Time Interval (TTI) based on a Hybrid Automatic Repeat Request (HARQ) process in an asynchronous code division multiple access (CDMA) communication system in which an uplink packet transmission is performed. In particular, the present invention relates to a method and apparatus for changing the TTI based on the HARQ process in a CDMA communication system that supports an Enhanced Uplink Dedicated transport Channel (E-DCH).
2. Description of the Related Art
An Enhanced Uplink Dedicated transport Channel (E-DCH) system is a system provided for improving the performance of packet transmission through the introduction of a new technology in an uplink communication in an asynchronous code division multiple access (CDMA) communication system. In order to improve the transmission efficiency in the recently introduced E-DCH system, an adaptive modulation and coding (hereinafter referred to as an “AMC”) system, an n-channel stop and wait HARQ (hereinafter referred to as an “n-channel SAW HARQ”) system and a Transmit Time Interval (TTI) and Node B control scheduling method may be used.
FIG. 1 is a view illustrating the operation of E-DCH.
Referring to FIG. 1, the reference numeral ‘110’ denotes a base station (hereinafter referred to as a “Node B”) that supports the E-DCH, and ‘101 to 104’ denote mobile terminals (hereinafter referred to as User Equipments (UE's)) for receiving/transmitting the E-DCH. Communication channels 111, 112, 113 and 114 provide a wireless link between the Node B 110 and the mobile terminals 101, 102, 103 and 104, respectively. The Node B 110 performs scheduling for each UE based on the channel environments of the UEs 101 to 104 that use the E-DCH. In order to improve the performance of the whole system, the Node B allocates a low data rate to UEs that are located far from the Node B, and allocates a high data rate to UEs that are near the Node B, without surpassing the wireless resource limit of the Node B.
Hereinafter, the basic transmitting/receiving procedure of the E-DCH will be explained with reference to FIG. 2. In FIG. 2, the reference numeral ‘202’ denotes a UE for transmitting the E-DCH, and ‘201’ denotes a Node B (i.e., a base station) to which the UE 102 belongs. The Node B 201 and the UE 202 perform an initial setting process for transmitting/receiving the E-DCH at step 203. This setting process comprises a process of transferring a message and so on through a dedicated transport channel. After the E-DCH setting is performed at step 203, the UE 202 informs the Node B 201 of scheduling information at step 204. The scheduling information at step 204 may be information about transmission power of the UE from which uplink channel information can be known, information about spare power that the UE can transmit, the amount of data stored on a buffer of the UE to be transmitted, etc. At step 204, the Node B 201 receives the scheduling information from the UE 202. Then, at step 211, the Node B 201 determines whether to perform scheduling of the E-DCH for the UE 202 based on the received scheduling information.
If the scheduling of the E-DCH for the UE 202 is determined, the Node B 201 generates scheduling allocation information for the UE 202. For reference, if a plurality of UEs belong to the Node B 201 and simultaneously request the E-DCH service, the Node B 201 should receive the scheduling information from the respective UEs. Additionally, the Node B may generate the scheduling allocation information for a specified UE based on the scheduling information received from the respective UEs. However, in the following description of the present invention, only one UE 202 will be considered for convenience. At step 205, the Node B 201 transmits the scheduling allocation information generated for the UE 202 to the UE 202. At that time, the scheduling allocation information comprises information about a data rate, transmission timing, and so on. The UE 202 that has received the scheduling allocation information at step 205 transmits the E-DCH using the scheduling allocation information at step 207, and simultaneously transmits a transport format resource indicator (hereinafter referred to as a “TFRI”) of the transmitted E-DCH to the Node B 201 together with the E-DCH at step 206. The Node B 201 that has received the E-DCH determines whether any error occurs in the TFRI or E-DCH at step 213. If an error occurs in either the TFRI or the E-DCH, the Node B 201 transmits negative acknowledge (NACK) information to the UE 202, and if no error occurs in the TFRI and the E-DCH, it transmits acknowledge (ACK) information to the UE 202 through an ACK/NACK channel at step 208. At that time, the conventional downlink Dedicated Physical Data Channel (DPDCH), downlink Dedicated Physical Control Channel (DPCCH), and so on, may be used as the ACK/NACK channel. Also, the ACK/NACK channel may be time-multiplexed with other channels, or may be defined as a separate channel.
Hereinafter, the n-channel SAW HARQ system will be explained in detail.
N-channel SAW HARQ system is a general term for indicating a system that has recently introduced the two following schemes in order to improve the efficiency of the typical SAW ARQ system.
First, a receiving part reduces the probability of error occurrence by temporarily storing data having an error and combining the data with a retransmitted portion of the corresponding data. This process is called a soft combining. The soft combining comprises two techniques—a chase combining (CC) and an incremental redundancy (IR).
In the CC, a transmitting part uses the same transmission format when it initially transmits the data and when it retransmits the data. If it is assumed that m symbols that comprise one coded block are transmitted during the initial transmission of data, the same number of symbols are transmitted during the retransmission of data. That is, during the initial transmission and the retransmission of data, the same coding rate is applied. The receiving part combines the initially transmitted data block with the retransmitted data block, performs a CRC operation using the combined data block, and confirms if an error occurs.
In the IR, different transmission formats are used during the initial transmission and retransmission of data. If it is assumed that n-bit user data is converted into m symbols through the channel coding, the transmitting part transmits only a part of the m symbols during its initial transmission of data, and then sequentially transmits the remaining part during its retransmission of data. That is, the coding rate of the initial transmission of data is different from that of the retransmission of data. The receiving part constructs a data block having a high coding rate by attaching the data block transmitted during the retransmission of data to the tail of the initially transmitted data block, and then performs an error correction. In the IR, the initially transmitted data block and the retransmitted data block are differentiated from each other by their version numbers. The initial transmission, the next transmission and the subsequent transmission are called version 1, version 2 and version 3, respectively, and the transmitting part can properly combine the initially transmitted data block with the retransmitted data block using the version information.
In the n-channel SAW HARQ system, the second system introduced in order to improve the efficiency of the conventional SAW ARQ system is as follows. In the conventional SAW ARQ system, the next packet can be transmitted only after the ACK signal of the previous packet is received. In the n-channel SAW HARQ system, a plurality of packets are successively transmitted without receiving any ACK signal to improve the efficiency of the wireless link. In the n-channel SAW HARQ, if n logic channels are set between the UE and the receiving part and identifies the channels by a specified time or channel number, the receiving part can recognize which channel the packet received at a certain time point belongs to, and thus can independently perform the HARQ process such as the reconstruction of the packets in the order of their reception, the soft combining of the corresponding packet, and so on.
FIG. 3 is a view illustrating the HARQ operation to be applied in the E-DCH. Here, it is assumed that the number of channels is 4, and four independent HARQ processes can be performed. The UE transmits packet data in the unit of a TTI. If the UE transmits HARQ process #1 301, HARQ process #1 301 reaches the Node B 201 after a specified propagation time (Tprop) 302 elapses. The Node B 201 performs a demodulation of the received data after it receives the data as long as the corresponding TTI 303. If no error occurs as a result of demodulation, the Node B generates the ACK signal 304 while if an error occurs, it generates the NACK. The time required for the Node B to receive the data and generate the ACK/NACK signal corresponds to TNBP 305, which varies according to the size of data and the characteristic of the receiver. The ACK/NACK signal transmitted by the Node B reaches the UE after a propagation time (Tprop) 306 elapses. The UE can calculate which frame the ACK/NACK response of the corresponding channel reaches in consideration of the above-described time. In other words, if the data is transmitted through HARQ process #1 301, the UE 202 can recognize that the ACK/NACK information received after the time period of ‘2XTprop+TNBP’ is the ACK/NACK signal for the HARQ process #1 301. However, the accurate time relation is determined by the above-described time and the maximum supportable number of HARQ channels. As illustrated in FIG. 3, the ACK/NACK signal of the HARQ process #1 is always transmitted to the frame of the time (TACK) 307. If the UE 202 receives the ACK signal for the HARQ process #1 at a determined time, it transmits a new packet for the next TTI that corresponds to the time 307, while if the UE 202 receives the NACK signal, it retransmits the data for the HARQ process #1 stored in the buffer using the above-described CC or IR method. Meanwhile, if the UE cannot receive the ACK or NACK signal from the designated frame, it determines that the packet transmission of the corresponding channel fails, and performs the retransmission process. The processing time of the UE taken for the initial transmission or retransmission of data after the ACK signal is received corresponds to TUEP 308 in FIG. 3.
The above-described HARQ operation is performed in the unit of a TTI, and for the E-DCH service, a short TTI for 2 ms and a long TTI for 10 ms have now been described.
In the case of using the long TTI, the existing R99 DPDCH structure can be used, but a delay is lengthened in comparison to the short TTI. In the case of using the short delay, the delay can be shortened, but a new physical layer channel is required because the TTI shorter than the existing DPCH is used and a separate signaling method is required for the existing Transport Format Combination Indicator. The simplest method that uses the short TTI is to add a new code channel, but this method has the drawback in that it increases a peak to average ratio. Since the short TTI and the long TTI have the respective advantages/drawbacks as described above, the use of the two TTIs according to the conditions for the E-DCH service would be able to increase the efficiency of the whole service. In the case in which the two TTIs are variably used according to the conditions of the UE or the Node B, the influence exerted on the HARQ should be considered.
As described above, the packet transmission according to the conventional method is performed in the unit of a TTI. Accordingly, if the corresponding TTI is changed, the data transmission/reception should be performed according to the changed TTI, and the HARQ operation that includes the corresponding ACK/NACK signal transmission/reception and the retransmission of the packet should be changed according to the changed TTI. That is, if the HARQ operation is not completed at the time point when the TTI is changed, the changed TTI becomes different from the TTI set at the initial transmission of the data. Accordingly, a need exists for a proper method for the TTI change in consideration of the HARQ operation is required.